1. FIELD OF THE INVENTION
The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to a method of processing a semiconductor substrate in forming a silicide by growing a thin metal film on the semiconductor substrate.
2. DESCRIPTION OF THE PRIOR ART
With an increase in the integration degree of a semiconductor device, the gate interconnection width and thickness decrease. Decreases in the width and thickness of gate interconnections and diffusion layers inevitably cause an increase in interconnection resistance, greatly influencing a delay in a circuit operation. In a micropatterned semiconductor device, therefore, a technique based on a refractory metal silicide has recently been used as a technique of attaining a decrease in resistance.
As a conventional method of forming a diffusion layer by using a silicide, a manufacturing method disclosed in Japanese Unexamined Patent Publication No. 4-34926 will be described with reference to FIGS. 1A to 1G. In this method, a silicon oxide film is formed on the upper surface of a silicon substrate before deposition of titanium as a refractory metal on the substrate to reduce the rate at which the silicide film consisting of titanium (to be simply referred to as the silicide film hereinafter) is formed, thereby preventing an unnecessary silicide from growing on an element isolation oxide film or an oxide film on a side wall of a gate electrode and causing short circuit in the device.
As shown in FIG. 1A, a field insulating film 42 for element isolation is formed on a p-type silicon substrate 41, and the gate electrode of a MOS transistor and a side wall spacer are formed in a region (not shown). As shown in FIG. 1B, after a spontaneous oxide film on an element region is completely removed by dilute hydrofluoric acid, a polysilicon film 45 is deposited. This polysilicon film is oxidized to form a silicon oxide film 44 having a thickness of about 10 nm on the element region, as shown in FIG. 1C. Thereafter, as shown in FIG. 1D, ion implantation and heat treatment are performed to form a source/drain region 43 of the MOS transistor. Further, as shown in FIG. 1E, the silicon oxide film 44 is etched to a thickness of about 5 nm with a dilute hydrofluoric acid. As shown in FIG. 1F, a titanium film 46 is formed on the entire surface of the resultant structure. Heat treatment based on lamp annealing is then performed for the resultant structure to form a silicide film 47, as shown in FIG. 1G. After this process, the remaining unreacted titanium is removed by an etching process using a solution mixture of ammonia and hydrogen peroxide which is generally used. The formed silicide film 47 undergoes lamp annealing again at a temperature of about 800.degree. C. to 900.degree. C. to cause phase transition to a C54 structure as a low-resistance silicide crystal structure.
Another conventional manufacturing method is disclosed in Japanese Unexamined Patent Publication No. 2-260630. The source/drain region of a MOS transistor is formed on a silicon substrate by a conventional manufacturing method. Thereafter, a spontaneous oxide film on an element region is completely removed with dilute hydrofluoric acid, and an oxide film having a thickness of 5 nm to 30 nm is formed by thermal oxidation. Titanium is deposited on the entire surface of the resultant structure, and heat treatment based on lamp annealing is performed to form a silicide film. After this process, the remaining unreacted titanium is removed by an etching process using a solution mixture of ammonia and hydrogen peroxide which is generally used. The formed silicide film 47 undergoes lamp annealing again at a temperature of about 800.degree. C. to 900.degree. C. to cause phase transition to a low-resistance crystal structure.
With the tendency toward an increase in the integration degree of a MOS transistor, a diffusion layer tends to form a shallower junction. With this tendency, demands have arisen for thinner silicide films. For thinner silicide films, several methods may be considered. Of these methods, a method of decreasing the thickness of a titanium deposition film itself is effective.
Conventionally, a titanium deposition film is thick, and hence a silicide film is also thick. For this reason, no serious problems have been posed regarding the state of the interface between a silicon film and a metal film in the early stage of silicide reaction. In a transistor having a size of half a micron or less, the thickness of a titanium deposition film needs to be 50 nm or less because of the relationship between the junction depth of a diffusion layer and the thickness of a silicide film. Consequently, a formed silicide film becomes thinner, the silicide film has uneven portions in terms of film thickness owing to the influence of a spontaneous oxide film at the interface between the silicon film and the metal film. Especially when the thickness of a titanium deposition film becomes 35 nm or less, the unevenness of the thickness of the silicide film causes agglomeration of the silicide film in heat treatment in the subsequent step. As a result, the diffusion layer resistance increases to cause variations in transistor characteristics.
In the prior art, an oxide film is removed with dilute hydrofluoric acid before deposition of titanium. As shown in FIG. 2, however, when about three hours have elapsed after the dilute hydrofluoric acid treatment, a spontaneous oxide film grows to such an extent that the formation of a silicide film is affected. When the thickness of a titanium deposition film is 35 nm, metal sputtering may be performed immediately after dilute hydrofluoric acid treatment to eliminate the influence of a spontaneous oxide film. On a manufacturing line for mass production, it is difficult to reliably ensure the time for metal sputtering within three hours after dilute hydrofluoric acid treatment.
In the above conventional technique, in forming an oxide film on a diffusion layer, variations in film thickness in forming a polysilicon film, variations in an etching process using dilute hydrofluoric acid, and the like occur. As a result, an oxide film formed on an element region varies in thickness within a silicon wafer plane. The formed silicide film also varies in thickness, resulting in variations in sheet resistance. In addition, when an oxide film is directly formed on a silicon substrate by thermal oxidation, the resultant film is a dense, rigid oxide film. This will interfere with the formation of a silicide film.